Possible sludged-up engine with sludge blocking the oil pick-up screen and/or oil drains in heads. Assuming you have been checking the oil level throughout this period and know that the oil level is good and the oil is still somewhat transparent. I would also pull the oil filter and examine it, looking for anything unusual. Just for grins, put a new one on, add a little make up oil and see if it makes any difference. Otherwise, I would be planning to pull the oil pan and see what it looks like inside.
Just curious, what oil and viscosity have you been using, and did you buy the truck new?